Time aligned group audio reproduction in narrowband and broadband networks

ABSTRACT

A method for synchronizing media reproduction across heterogeneous networks is presented. The networks include end-to-end IP broadband and narrowband simulcast networks that contain broadband and narrowband devices associated with a common communications group. A controller in the networks determines delay times for reproduction of a media stream across devices in the networks and establishes the longest delay time. The longest delay time is used to calculate appropriate transmission and reproduction timestamps to permit the devices to reproduce the provided media stream in synchronization. Narrowband base stations repeat the media stream at the time specified by a transmission timestamp, and broadband end devices reproduce the media stream at the time specified by a reproduction timestamp. By synchronizing the presentation time, the devices present the media at substantially the same time and are granted fair rights to communicate with one another.

TECHNICAL FIELD

The present application relates to heterogeneous networks. Inparticular, the application relates to simultaneous reproduction of anaudio signal in heterogeneous networks.

BACKGROUND

Group-directed communications are commonplace in enterprise and publicsafety communication systems. With regard to voice communications, oneend device directs an audio stream to a given group (i.e. a “talkgroup”)of receiving end devices. These receiving end devices reproduce theaudio stream through an amplified speaker. The manner in which thereceiving end devices are used usually results in the reproduced soundbeing audible to people other than merely the intended recipient.Typically, the receiving end devices are often located near each other,causing their associated listeners to hear the same audio streamsimultaneously reproduced by multiple end devices. This is particularlytrue in public safety uses, in which personnel often respond toincidents in a group and this group (or a subset thereof) may be locatedin the same local area for an extended period of time.

In order to ensure the audio stream is intelligible to the intendedlisteners in such an environment, it is desirable for collocated devicesto reproduce the audio stream in a time synchronized fashion. In otherwords, all amplified speakers in the collocated devices should reproducethe same audio waveform at roughly the same instant in time. In general,a temporal offset of at most 30 ms between multiple audible speakersreproducing the same waveform is virtually undetectable to mostlisteners. Modern wireless voice communication systems achievesynchronized presentation of group-directed audio through anover-the-air simulcast of circuit-switched audio at multipletransmitting sites. Dense populations of collocated end devices thusreceive the same over-the-air signal at roughly the same instant intime.

Such methods of synchronized presentation work well for the specializedhomogeneous narrowband circuit-switched wireless radio networkstypically used in the current generation of enterprise and public safetycommunication systems. However, the next generation of suchcommunication systems is likely to span multiple narrowbandcircuit-switched and broadband packet-switched Radio Area Network (RAN)technologies with wholly different methods of synchronization. Examplecircuit-switched narrowband RAN technologies include 25 kHz, 12.5 kHz,or 6.25 kHz equivalent FDMA or TDMA air interfaces (e.g. Project 25,TETRA, DMR). Example packet-switched broadband RAN technologies includeLTE, UMTS, EVDO, WiMAX, and WLAN air interfaces. Without a mechanism tosynchronize media reproduction in a communication system comprised ofheterogeneous RAN technologies, end devices connected to thecircuit-switched narrowband RAN and end devices connected to thepacket-switched broadband RAN would reproduce the same audio waveform inan autonomous fashion with respect to one another. This cacophony ofmisaligned sound results in unintelligible audio communication wheremultiple narrowband and broadband end devices are collocated.

Additionally, half-duplex group communication systems provide amechanism to ensure equitable speaking rights on a given sharedcommunication resource such as a channel or “talkgroup.” To providethis, the floor (i.e. the right to broadcast) is typically granted tothe first device to make an appropriate request. During a half-duplexgroup conversation, listeners wait for the current audio stream tofinish before initiating a new floor request. If the floor is granted tothe first requester, it is desirable that all listeners be given theopportunity to request the floor at the same instant. This can beachieved if the preceding audio stream ends at the same time for alllisteners.

In addition to potential intelligibility problems, without synchronizedaudio reproduction, the same audio stream will terminate at differenttimes for listeners whose end devices are connected via different RANtechnologies. Since floor control is typically granted to the firstrequester, end devices whose reproduced audio stream is lagging are notgiven equal rights for floor acquisition. Thus, it is desirable toprovide a mechanism to synchronize audio reproduction across end devicesoperating on heterogeneous RANs.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 illustrates one embodiment of a system.

FIG. 2 illustrates another embodiment of a system.

FIG. 3 illustrates the calculated time delays in the embodiment of FIG.1.

FIG. 4 illustrates the calculated time delays in the embodiment of FIG.2.

DETAILED DESCRIPTION

Coordinated media (e.g. audio) reproduction across differentcommunication networks, such as narrowband (hereinafter referred to asNB) simulcast and broadband (hereinafter referred to as BB) networks, ispresented.

The presentation time of media to a heterogeneous group of end devicescontaining, for example, NB End Devices and BB End Devices, is timealigned such that an audio signal, for example, is reproduced at roughlythe same time for all of the End Devices. This synchronization providescoherent group audio reproduction, which allows multiple listeners tohear the same audio signal from heterogeneous End Devices in the samephysical vicinity without the interference caused by misaligned andoverlapping audio streams. In addition, the synchronization ensures fairfloor access in half-duplex communication systems as each listener isgiven the opportunity to attempt floor acquisition at the same time.

One embodiment of a group communication system containing multipleheterogeneous RAN technologies is depicted in FIG. 1. The embodiment ofFIG. 1 includes a NB simulcast RAN (hereinafter referred to as NB RAN102), which could be, for example, part of a Project 25 compliant PTTsystem. FIG. 1 also includes a BB RAN 103, which could be, for example,part of an OMA PoC (Open Mobile Alliance Push-to-talk over Cellular)compliant PTT system. Integrated together, they form a singlecommunication System 100. Specifically, the System 100 shown in FIG. 1includes a NB/BB Controller 104, a NB Time Source 110, a BB Time Source111, NB Base Stations 120, BB Base Stations 121, NB End Devices 130, 132and BB End Devices 133. The NB/BB Controller 104 has the ability toindependently delay a group-directed audio signal to the NB RAN 102 orBB RAN 103, thereby accommodating the NB or BB End Device 132, 133 whichexhibits a statistically significant, e.g. worst case, delay of an audiosignal measured from the NB/BB Controller 104 to its reproduction in theNB or BB End Device 132, 133. In practice, the delay of an audio signalfrom the NB/BB Controller 104 to its reproduction in BB End Devices 133,only one of which is shown for convenience, is typically significantlylonger with respect to the same audio signal transmitted to andreproduced by NB End Devices 132.

Each of the NB and BB End Devices 130, 132, 133 is a user device thathas a transmitter and receiver (not shown). Although mobile NB or BB EndDevices are described, at least some of the NB or BB End Devices 130,132, 133 may be geographically fixed. The NB or BB End Devices 130, 132,133 communicate with other NB or BB End Devices 130, 132, 133 via anassociated NB Base Station 120 or BB Base Station 121, respectively, aswell as other not depicted interconnections of the System 100 andassociated functions including the NB/BB Controller 104. Note that whileonly one intermediary (illustrated as a Base Station) is shown betweeneach of the NB and BB End Devices 130, 132, 133 and the NB/BB Controller104 for convenience, one or more intermediaries of different types maybe inserted depending on the specific RAN technology deployed. AlthoughNB Base Stations 120 or BB Base Stations 121 and other intermediariesmay be mobile (handsets or vehicle mounted), such elements alternativelymay be geographically fixed. Each of the NB and BB End Devices 130, 132,133 also has a speaker (not shown) through which the End Device providesacoustic reproduction of audio to the user, in addition to othercircuitry and input/output mechanisms.

The NB End Devices 130, 132 communicate through the NB RAN 102. Examplesof such NB End Devices 130, 132 include portable and mobile NB radios orany other End Device which connects, in a wireless fashion, to the NBRAN 102. These NB End Devices 130, 132 are connected to the NB/BBController 104 via NB Base Stations 120. Referring to FIG. 1, one of theNB End Devices 130 requests and is granted the floor (i.e. the right tospeak on a given communication resource) from a floor controller (notshown) of System 100. This NB End Device 130 transmits an audio stream(hereinafter referred to as NB

Uplink Audio Stream 140) to the NB/BB Controller 104 via the NB BaseStation 120. The other NB End Devices 132 receive the repeated audiostream (hereinafter referred to as NB Downlink Audio Stream 144) fromthe NB/BB Controller 104 via one or more NB Base Stations 120.

The BB End Devices 133 communicate through the BB RAN 103. Examples ofsuch BB End Devices 133 include cell phones, PDAs, laptop computers, orany other End Device which connects, in a wired or wireless fashion, tothe BB RAN. The BB End Devices 133 are connected to the NB/BB Controller104 through BB Base Stations 121. Only one BB End Device 133 and one BBBase Station 121 are shown in FIG. 1 for clarity. The BB End Devices 133receive the audio stream (hereinafter referred to as BB Downlink AudioPackets 145) from the NB/BB Controller 104 via BB Base Stations 121.Although not illustrated in FIG. 1, BB End Devices 133 along with othercomponents of System 100 not shown in FIG. 1 (e.g. a wired voicedispatch console) are equally capable of requesting the floor andtransmitting an audio stream to the NB/BB Controller 104.

The NB/BB Controller 104 is a combined NB simulcast and BB controllerthat is responsible for duplicating and routing audio streams to all NBand BB End Devices 130, 132, 133 affiliated to the same logical group.The NB and BB End Devices 130, 132, 133 join a group, for example, byturning a physical knob on the device to select a particular logical“talkgroup” or “channel.”

In the NB RAN 102, the NB/BB Controller 104 synchronizes a simulcasttransmission of the NB Downlink Audio Stream 144 at the appropriate NBBase Stations 120 by specifying a transmission timestamp (hereinafterreferred to as NB_(TransmissionTimestampN)) for each audio frame(hereinafter referred to as Audio_(FrameN)) contained in the NB DownlinkAudio Stream 144. NB_(TransmissionTimestampN) is expressed in valuesrelative to a common clock reference (hereinafter referred to as NB TimeSource 110) known to the NB/BB Controller 104 and the NB Base Stations120. The NB/BB Controller 104 and the NB Base Stations 120 contain veryhigh precision, nanosecond-accurate, internal clocks (hereinafterreferred to as NB Clocks 114) synchronized to a common NB Time Source110, e.g. the Global Positioning Satellite (GPS) system 1 PPS (Pulse PerSecond), via NB Clock Signal 112. When the NB/BB Controller 104 receivesthe NB Uplink Audio Stream 140 from one of the NB End Devices 130 via aNB Base Station 120, the NB/BB Controller 104 repeats the series ofreceived Audio_(FrameN)s, along with an associated series ofNB_(TransmissionTimestampN)s, in NB Downlink Audio Stream 144 to theappropriate NB Base Stations 120. Upon receiving NB Downlink AudioStream 144, the participating NB Base Stations 120 wait until theirsynchronized NB Clocks 114 are exactly equal toNB_(TransmissionTimestampN) specified for a given Audio_(FrameN). Atthat instant in time, the participating NB Base Stations 120simultaneously repeat Audio_(FrameN) to all NB End Devices 132affiliated to the group to which the audio stream is directed.

However, the combined NB/BB Controller 104 does not merely repeat thesame NB Downlink Audio Stream 144 provided to NB Base Stations 120 to BBEnd Devices 133 (by way of BB Base Stations 121). One reason for this isthat the timing and synchronization mechanisms used in the NB RAN 102are typically quite different from those available in the BB RAN 103.Although it is theoretically possible to extend the same time-stamped NBDownlink Audio Stream 144 to BB End Devices 133 if a similar timingmechanism (e.g. a very high precision GPS-locked clock) were disposed inthe BB End Devices 133, providing the BB End Devices 133 with suchequipment may be impracticable at least due to cost, size, and locationconcerns. Additionally, the values of NB_(TransmissionTimestampN)present in NB Downlink Audio Stream 144 specify a transmission time forAudio_(FrameN). This transmission time is not inclusive of the timeneeded to process and acoustically reproduce Audio_(FrameN). Since theamount of time used to perform these functions likely differs amongst NBand BB End Devices, BB End Devices 133 do not possess enough informationto synchronize their audio reproduction with that of NB End Devices 132.

The NB/BB Controller 104 and the BB End Devices 133 shown in FIG. 1contain moderate precision, millisecond-accurate, internal clocks(hereinafter referred to as BB Clocks 115) locked to a common BB TimeSource 111, e.g. a time-of-day clock, via BB Clock Signal 113. Unlikesimulcast transmission, which is achieved using nanosecond-accuratetiming mechanisms present in participating NB Base Stations 120, timealigned reproduction of audio can be achieved using merely themillisecond-accurate timing mechanisms present in participating BB EndDevices 133. The NB/BB Controller 104 specifies a reproduction timestamp(hereinafter referred to as BB_(ReproductionTimestampN)) for one or moreAudio_(FrameN)(s) contained in BB Downlink Audio Packets 145 repeated tothe appropriate BB End Devices 133 (i.e., the BB End Devices 133 thathave selected the channel and joined the group to which theAudio_(FrameN)(s) are transmitted). BB Downlink Audio Packets 145 areformatted, for example, using the Real-time Transport Protocol (RTP).BB_(ReproductionTimestampN)s embedded in BB Downlink Audio Packets 145are relative to the common BB Time Source 111 and inform the BB EndDevices 133 as to the exact time the associated Audio_(FrameN) is to beacoustically reproduced. Upon receiving a BB Downlink Audio Packet 145,the participating BB End Device 133 waits until their synchronized BBClocks 115 are exactly equal to BB_(ReproductionTimestampN) specifiedfor a given Audio_(FrameN). At that instant in time, the participatingBB End Devices 133 simultaneously reproduce Audio_(FrameN).

The theoretical delay from the time at which NB/BB Controller 104 sendsan audio signal until the time at which NB End Devices 132 reproducethat audio signal is calculated by a processor (not shown) in the NB/BBController 104 prior to the NB/BB Controller 104 receiving a NB UplinkAudio Stream 140. Recall that NB Base Stations 120 and the NB/BBController 104 contain NB Clocks 114 synchronized to the same NB TimeSource 110. To measure the signal propagation delay from the NB/BBController 104 to each NB Base Station 120, the NB/BB Controller 104samples the value of NB Clock 114, and sends a time-stamped messagecontaining this value (hereinafter referred to as NB Time MeasurementPackets 146) to each NB Base Station 120. Upon receiving NB TimeMeasurement Packet 146, a NB Base Station 120 subtracts the embeddedtimestamp from its NB Clock 114 to derive the one-way signal propagationdelay (hereinafter referred to as NB_(PropagationDelayBaseSiteN))between the NB/BB Controller 104 and the NB Base Station 120.NB_(PropagationDelayBaseSiteN) is then sent back to the NB/BB Controller104 where it is recorded in a memory (not shown). All suchNB_(PropagationDelayBaseSiteN) measurements to each NB Base Station 120are then compared and a statistically significant (e.g. worst case, 99%worst case, 95% worst case, 90% worst case) one-way propagation delay(hereinafter referred to as NB_(PropagationDelay)) from the NB/BBController 104 to all NB Base Stations 120 is recorded in the NB/BBController 104. The wireless propagation delay between the NB BaseStation 120 and the NB End Devices 132 is comparatively negligible. Thestatistically significant delay from the time an audio frame is sentfrom the NB/BB Controller 104 to the time the audio signal it containsis acoustically reproduced by the speaker in a NB End Device 132 is thencalculated as:

NB _(ReproductionDelay) =NB _(PropagationDelay) +NB_(DeviceProcessingDelay)

where NB_(DeviceProcessingDelay) is the known time to process (e.g.,demodulate, error-correct, and decode) the audio signal in the NB EndDevices 132. NB_(DeviceProcessingDelay) is measured or estimated priorto the NB End Devices 132 being shipped and device-to-device variationis comparatively negligible. NB_(ReproductionDelay) may be periodicallyrecalculated by NB/BB Controller 104, which permits modification ofNB_(ReproductionDelay) as participating NB Base Stations 120 are addedto or removed from the NB RAN 102.

Similarly, the theoretical delay from the time at which NB/BB Controller104 sends an audio signal until the time at which BB End Devices 133reproduce that audio signal is calculated by the processor in the NB/BBController 104 prior to it receiving NB Uplink Audio Stream 140. Recallthat BB End Devices 133 and the NB/BB Controller 104 contain BB Clocks115 synchronized to the same BB Time Source 111. To measure the signalpropagation delay from the NB/BB Controller 104 to each BB End Device133, the NB/BB Controller 104 samples the value of BB Clock 115, andsends a time-stamped message containing this value (hereinafter referredto as BB Time Measurement Packets 147) to a representative set, e.g.all, of the BB End Devices 133. Upon receiving BB Time MeasurementPacket 147, the BB End Device 133 subtracts the embedded timestamp fromits BB Clock 115 to derive the one-way signal propagation delay betweenthe NB/BB Controller 104 and the BB End Device 133 (hereinafter referredto as BB_(PropagationDelayDeviceN)). All suchBB_(PropagationDelayDeviceN) measurements are then compared and thestatistically significant one-way propagation delay (hereinafterreferred to as BB_(PropagationDelay)) from the NB/BB Controller 104 tothe representative set of BB End Devices 133 is recorded in the NB/BBController 104. The statistically significant delay from the time anaudio frame is sent from the NB/BB Controller 104 to the time the audiosignal it contains is acoustically reproduced by the speaker in the BBEnd Device 133 is then calculated as:

BB _(ReproductionDelay) =BB _(PropagationDelay) +BB_(DeviceProcessingDelay)

where BB_(DeviceProcessingDelay) is the known time to process (e.g.,demodulate, error-correct, de-jitter, and decode) audio packets in theBB End Devices 133. Similar to NB_(DeviceProcessingDelay),BB_(DeviceProcessingDelay) is measured or estimated prior to BB EndDevice 133 being shipped and device-to-device variation is againcomparatively negligible. As above, BB_(ReproductionDelay) may beperiodically recalculated by NB/BB Controller 104, which permitsmodification of BB_(ReproductionDelay) as participating BB End Devices133 are added to or removed from the BB RAN 103. A diagram of the timedelays described above in relation to the embodiment of FIG. 1 is shownin FIG. 3.

As above, the NB/BB Controller 104 specifies aNB_(TransmissionTimestampN) to NB Base Stations 120 and aBB_(ReproductionTimestampN) to BB End Devices 133 for eachAudio_(FrameN) repeated. To facilitate this, the NB/BB Controller 104calculates the delay from a starting time 0, in units of the NB Clock114, at which time the first Audio_(Frame0) is to be simulcast by NBBase Stations 120 (hereinafter referred to asNB_(TransmissionTimestampDelay)) and the delay from the same startingtime 0, in units of the BB Clock 115, at which time the firstAudio_(Frame0) is to be reproduced by BB End Devices 133 (hereinafterreferred to as BB_(ReproductionTimestampDelay)). In an ordinary NB RAN102, NB_(TransmissionTimestampDelay) is calculated by the NB/BBController 104 to be equal to NB_(PropagationDelay). Here, however,NB_(TransmissionTimestampDelay) and NB_(PropagationDelay) are calculatedvia the following algorithm:

IF BB_(ReproductionDelay*)≦NB_(ReproductionDelay) THEN:

NB _(TransmissionTimestampDelay) =NB _(ReproductionDelay) −NB_(DeviceProcessingDelay);

AND

BB_(ReproductionTimestampDelay)=NB_(ReproductionDelay*);

ELSE IF BB_(ReproductionDelay*)>NB_(ReproductionDelay) THEN:

NB _(TransmissionTimestampDelay) =BB _(ReproductionDelay*) −NB_(DeviceProcessingDelay);

AND

BB_(ReproductionTimestampDelay)=BB_(ReproductionDelay);

where BB_(ReproductionDelay*) is BB_(ReproductionDelay) in units of NBClock 114, and NB_(ReproductionDelay*) is NB_(ReproductionDelay) inunits of BB Clock 115. This translation between clock units is possible,since NB/BB Controller 104 knows the respective frequencies (e.g. 1 kHz,1 MHz, 1 GHz) and relationship (i.e. at a given instant in time, it cansample both clocks) of both NB Clock 114 and BB Clock 115.

NB_(TransmissionTimestampDelay) and BB_(ReproductionTimestampDelay) maybe stored on a per-group basis in a periodically-updated database in theNB/BB Controller 104. This permits the NB/BB Controller 104 to adjustthese values whenever a new NB or BB End Device 130, 132, 133 joins orleaves the group if the particular End Device statistically affectsthese calculated delay values in a significant way (e.g., greater than1%, 2%, 5%, 10%, etc.). When a new End Device joins a particular group,the NB/BB Controller 104 determines its NB_(ReproductionDelay) orBB_(ReproductionDelay) and recalculates NB_(TransmissionTimestampDelay)or BB_(ReproductionTimestampDelay) if determined appropriate. Thus,although the System 100 may contain many End Devices, the NB/BBController 104 is able to adjust the NB_(TransmissionTimestampDelay) andBB_(ReproductionTimestampDelay) to account for only those End Devicesthat are to reproduce a given audio signal (e.g. that are present on thechannel and joined to a particular group). In addition, if desired, theNB/BB Controller 104 can calculate and store delays on a per-End Devicebasis, instead of on a per-group basis. Doing so permits the NB/BBController 104 to, for example, reduce the delay of the audio signal tothe reproducing End Devices if the transmitting End Device is also theEnd Device which exhibits the longest propagation delay.

When the first Audio_(Frame0) from NB Uplink Audio Stream 140 arrives atthe NB/BB Controller 104, the NB/BB Controller 104 immediately samplesboth the synchronized NB Clock 114 (hereinafter referred to asNB_(TimestampStart)) and the synchronized BB Clock 115 (hereinafterreferred to as BB_(TimestampStart)).

Each Audio_(FrameN) in NB Downlink Audio Stream 144 contains aNB_(TransmissionTimestampN) which is calculated per the followingalgorithm:

NB _(TransmissionTimestampN) =NB _(TimestampStart) +NB_(TransmissionTimestampDelay)+(Audio_(FrameTime) *N)

where Audio_(FrameTime) is the duration of audio, specified in units ofthe NB Clock 114, contained in each Audio_(FrameN) of the NB DownlinkAudio Stream 144. In the following example, each Audio_(FrameN) contains180 milliseconds of audio (i.e. Audio_(FrameTime)=180). It is understoodthat Audio_(FrameTime) could differ (e.g., 20 milliseconds, 60milliseconds) based on the system configuration and types of RANtechnologies employed.

For simplicity, assume that NB Clock 114 units are represented in unitsof milliseconds. Therefore:

NB _(TransmissionTimestamp0) =NB _(TimestampStart) +NB_(TransmissionTimestampDelay)+(180*0)

This continues:

NB _(TransmissionTimestamp1) =NB _(TimestampStart) +NB_(TransmissionTimestampDelay)+(180*1)

NB _(TransmissionTimestamp2) =NB _(TimestampStart) +NB_(TransmissionTimestampDelay)+(180*2)

The NB Base Stations 120 receiving NB Downlink Audio Stream 144 followordinary simulcast behavior, waiting until their NB Clocks 114 are equalto the specified NB_(TransmissionTimestampN) before transmitting thecorresponding Audio_(FrameN) to NB End Devices 132.

Each Audio_(FrameN) in BB Downlink Audio Packets 145 contains aBB_(ReproductionTimestampN) which is calculated per the followingalgorithm:

BB _(ReproductionTimestampN) =BB _(TimestampStart) +BB_(ReproductionTimestampDelay)+(Audio_(FrameTime) *N)

where Audio_(FrameTime) is the duration of audio, specified in units ofthe BB Clock 115, contained in each Audio_(FrameN) of the BB DownlinkAudio Packets 145.

The BB End Devices 133 receiving BB Downlink Audio Packets 145 by way ofBB Base Stations 121 wait until their BB Clocks 115 are equal to theBB_(ReproductionTimestampN) before acoustically reproducing theassociated Audio_(FrameN). The BB End Devices 133 perform decryption anddecompression to prepare the packet contents such that the audiowaveform can be presented to the listener at the time indicated byBB_(ReproductionTimestampN). If, for whatever reason, the BB End Device133 is late reproducing a particular Audio_(FrameN), it may employtechniques such as time compression to align with futureBB_(ReproductionTimestamps) embedded in Downlink BB Audio Packets 145.The term “late” may be set by an arbitrary threshold (hereinafterreferred to as BB_(ReproductionThreshold)) of 180 milliseconds, forexample. If BB_(ReproductionThreshold) is exceeded, the packet(s) may beskipped and audio reproduction may be started on time with subsequentpackets.

Another embodiment of the heterogeneous communication system is shown inFIG. 2. This System 200 includes a NB Controller 204, a BB Controller205, a NB Time Source 210, a BB Time Source 211, NB Base Stations 220,BB Base Stations 221, NB End Devices 230, 232, and BB End Devices 233. Acommon BB Time Source 211 is used to synchronize the BB Clock 215 in BBController 205 and BB End Device 233. A common NB Time Source 210 isused to synchronize the NB Clock 214 in NB Controller 204, BB Controller205, and NB Base Stations 220. Although only NB End Device 230 is shownto request the floor and transmit audio, it is understood that BB EndDevices 233 are equally capable of such behavior. In such cases, theaudio stream from a BB End Device 233 is first forwarded to NBController 204 such that it may be processed in a manner similar to thatof NB Uplink Audio Stream 240.

In the embodiment of FIG. 1, a single NB/BB Controller 104 repeats NBUplink Audio Stream 140 to both the NB and BB End Devices 132, 133through NB and BB RANs 102, 103 respectively. In the embodiment of FIG.2, however, a NB Controller 204 repeats NB Uplink Audio Stream 240 tothe NB End Devices 232 through NB RAN 202 and a separate BB Controller205 repeats NB Uplink Audio Stream 240 to the BB End Devices 233 throughBB RAN 203. In this embodiment, the NB Controller 204 treats the BBController 205 similar to another NB Base Station 220. Thus, the NBController 204 repeats NB Uplink Audio Stream 240 as NB Downlink AudioStream 244 to the BB Controller 205. Doing so essentially permits anordinary NB simulcast controller in NB RAN 202 to be used as the NBController 204. The BB Controller 205, upon receiving NB Downlink AudioStream 244 from NB Controller 204, reformats and repeats the audiostream as BB Downlink Audio Packets 245 to the BB End Devices 233 by wayof BB Base Stations 221.

Similar to the embodiment of FIG. 1, the BB Controller 205 periodicallymeasures BB_(PropagationDelayDeviceN) from the BB Controller 205 to arepresentative set, e.g. all, of the BB End Devices 233 using the BBTime Measurement Packets 247. As before, all suchBB_(PropagationDelayDeviceN) measurements to each BB End Device 233 arethen compared and a statistically significant (e.g. worst case, 99%worst case, 95% worst case, 90% worst case) one-way propagation delayfrom the BB Controller 205 to all BB End Devices 233 is recorded asBB_(PropagationDelay). The statistically significant delay from the timean audio frame is sent from the BB Controller 205 to the time the audiosignal it contains is acoustically reproduced by the speaker in the BBEnd Device 233 is then calculated as:

BB _(ReproductionDelay) =BB _(PropagationDelay) +BB_(DeviceProcessingDelay)

where BB_(DeviceProcessingDelay) is the known time to process (e.g.,demodulate, error-correct, de-jitter, and decode) audio packets in theBB End Devices 233. BB_(DeviceProcessingDelay) is measured or estimatedprior to BB End Device 233 being shipped and device-to-device variationis again comparatively negligible. As in the embodiment of FIG. 1,BB_(ReproductionDelay) may be periodically recalculated by BB Controller205, which permits modification of BB_(ReproductionDelay) asparticipating BB End Devices 233 are added to or removed from the BB RAN203.

The NB Controller 204 periodically measuresNB_(PropagationDelayBaseSiteN) from the NB Controller 204 to each NBBase Station 220 using the NB Time Measurement Packets 246. In contrastto the embodiment of FIG. 1, however, the NB Controller 204 depicted inthe embodiment of FIG. 2 considers BB Controller 205 to be another NBBase Station 220. As with all other NB Base Stations 220, the NBController samples its NB Clock 214, and sends a time-stamped messagecontaining this value to BB Controller 205. Upon receiving NB TimeMeasurement Packet 246, BB Controller 205 subtracts the embeddedtimestamp from its NB Clock 214 to derive the one-way signal propagationdelay (hereinafter referred to as NB_(PropagationDelayBBController))between the NB Controller 204 and the BB Controller 205. Unlike theoperation of other NB Base Stations 220, however, the BB Controller 205does not merely return NB_(PropagationDelayBBcontroller) back to NBController 204. Instead, BB Controller 205 calculates a newNB_(PropagationDelayBBcontroller†) per the following algorithm:

NB _(PropagationDelayBBController†) =NB _(PropagationDelayBBcontroller)+BB _(ReproductionDelay*) −NB _(DeviceProcessingDelay);

where NB_(DeviceProcessingDelay) is the known time, in units of NB Clock214, to process (e.g., demodulate, error-correct, and decode) the audiosignal in NB End Devices 232. BB_(ReproductionDelay*) is theBB_(ReproductionDelay) in units of NB Clock 214. OnceNB_(PropagationDelayBBcontroller†) is calculated, BB Controller 205returns this value to NB Controller 204 in a NB Time Measurement Packet246.

All NB_(PropagationDelayBasesiteN) measurements to each NB Base Station220 along with NB_(PropagationDelayBBcontroller†) as calculated aboveare then compared and a statistically significant (e.g. worst case, 99%worst case, 95% worst case, 90% worst case) one-way propagation delay(hereinafter referred to as NB_(PropagationDelay)) from the NBController 204 to all NB Base Stations 220 and BB Controller 205 isrecorded in the NB Controller 204. The wireless propagation delaybetween the NB Base Station 220 and the NB End Devices 232 iscomparatively negligible. The statistically significant delay from thetime an audio frame is sent from the NB Controller 204 to the time theaudio signal it contains is acoustically reproduced by the speaker inthe NB End Device 232 is then calculated as:

NB _(ReproductionDelay) =NB _(PropagationDelay) +NB_(DeviceProcessingDelay)

where NB_(DeviceProcessingDelay) is the known time to process (e.g.,demodulate, error-correct, and decode) the audio signal in the NB EndDevices 232. NB_(DeviceProcessingDelay) is measured or estimated priorto the NB End Devices 232 being shipped and device-to-device variationis again comparatively negligible. Unlike the embodiment of FIG. 1,NB_(ReproductionDelay) is also inclusive of theNB_(PropagationDelayBBcontroller†) as reported by BB Controller 205(which is itself inclusive of BB_(ReproductionDelay)). In this way, a NBDownlink Audio Stream 244 simulcast by NB Base Stations 220 may bedelayed to ensure acoustic alignment of NB End Devices 232 to BB EndDevices 233. NB_(ReproductionDelay) may be periodically recalculated byNB Controller 204, which permits modification of NB_(ReproductionDelay)as participating NB Base Stations 220 or BB End Devices 233 are added toor removed from the System 200. A diagram of the time delays describedabove in relation to the embodiment of FIG. 2 is shown in FIG. 4.

As in the embodiment of FIG. 1, the NB Controller 204 provides aNB_(TransmissionTimestampN) to NB Base Stations 220 for eachAudio_(FrameN) embedded in NB Downlink Audio Stream 244. To facilitatethis, NB Controller 204 calculates NB_(TransmissionTimestampDelay) whichrepresents the delay from a starting time 0, in units of the NB Clock214, at which the first Audio_(Frame0) is to be simulcast to NB BaseStations 220 and BB Controller 205. As in an ordinary NB RAN 202, NBController 204 assigns NB_(TransmissionTimestampDelay) as follows:

NB_(TransmissionTimestampDelay)=NB_(PropagationDelay)

Similar to the embodiment of FIG. 1, NB End Device 230 transmits NBUplink Audio Stream 240 to NB Controller 204 through a NB Base Station220. When the first Audio_(Frame0) from NB Uplink Audio Stream 240arrives at the NB Controller 204, the NB Controller 204 immediatelysamples the synchronized NB Clock 214 (hereinafter referred to asNB_(TimestampStart)).

Each Audio_(FrameN) in NB Downlink Audio Stream 244 will contain aNB_(TransmissionTimestampN) which is calculated per the followingalgorithm:

NB _(TransmissionTimestampN) =NB _(TimestampStart) +NB_(TransmissionTimestampDelay)+(Audio_(FrameTime) *N)

where Audio_(FrameTime) is the duration of audio, specified in units ofthe NB Clock 214, contained in each Audio_(FrameN) of the NB DownlinkAudio Stream 244.

The NB Base Stations 220 receiving NB Downlink Audio Stream 244 followordinary simulcast behavior, waiting until their NB Clocks 214 are equalto the NB_(TransmissionTimestampN) before broadcasting Audio_(FrameN) toNB End Devices 232.

The BB Controller 205 also receives NB Downlink Audio Stream 244 withembedded NB_(TransmissionTimestampN)s for each Audio_(FrameN). As in theembodiment of FIG. 1, BB Controller 205 provides aBB_(ReproductionTimestampN) to BB End Devices 233 for eachAudio_(FrameN) in BB Downlink Audio Packets 245. The BB Controller 205calculates BB_(ReproductionTimestampN) for each Audio_(FrameN) receivedin NB Downlink Audio Stream 244 as follows:

BB _(ReproductionTimestampN) =NB _(TransmissionTimestampN) *+NB_(DeviceProcessingDelay*)

where NB_(TransmissionTimestampN*) is the receivedNB_(TransmissionTimestampN) in units of BB Clock 215, andNB_(DeviceProcessingDelay*) is the NB_(DeviceProcessingDelay) in unitsof BB Clock 215. This translation between clock units is possible, sinceBB Controller 205 knows the respective frequencies (e.g. 1 kHz, 1 MHz, 1GHz) and relationship (i.e. at a given instant in time, it can sampleboth clocks) of both NB Clock 214 and BB Clock 215. As in the embodimentof FIG. 1, NB_(DeviceProcessingDelay) is measured or estimated prior tothe NB End Devices 232 being shipped and device-to-device variation isagain comparatively negligible.

The BB End Devices 233 receiving BB Downlink Audio Packets 245 by way ofBB Base Stations 221 wait until their BB Clocks 215 are equal to theBB_(ReproductionTimestampN) before acoustically reproducingAudio_(FrameN). The BB End Devices 233 perform decryption anddecompression to prepare the packet contents such that the audiowaveform can be presented to the listener at the time indicated byBB_(ReproductionTimestampN).

In the case of either of the two embodiments presented, certain rareconditions may lead to excessively long measured values ofNB_(PropagationDelayBasesiteN) and BB_(PropagationDelayDeviceN). In suchcases, these values are not representative of the vast majority ofsimilarly measured delays. The NB and/or BB Controllers can take thisinto account by discarding those delays that are in a preset percentileof the longest delays measured (e.g. >95%, >98%, >99%). This measurementis calculated by ordering all of the NB_(PropagationDelayBasesiteN)measurement values into an ordered set from minimum to maximum value.If, for example, the worst 90% NB_(PropagationDelayBasesiteN)measurement value is to be selected, the value in that ordered set whoseindex is 0.9 times the number of values in the set is chosen. If, forexample, the worst 95% NB_(PropagationDelayBasesiteN) measurement valueis to be selected, the value in that ordered set whose index is 0.95times the number of values in the set is chosen. This same method can beapplied to the measured BB_(PropagationDelayDeviceN) values. Note anyother statistical measure (e.g. time delays greater than two or threestandard deviations from the mean delay time) can alternatively be used.This measure provides a method of filtering out the extreme delay casesfrom greatly increasing the overall audio reproduction delay experiencedby all of the End Devices affiliated to a given group at the possibleunderstood cost of occasional overlapping audio and/or floor acquisitiondifficulty.

Although audio signals have been discussed, media signals other thansolely audio signals (e.g. text, device control, video) can also becoordinated using the above technique. In addition, although only NBsimulcast and BB RANs were described above, any set of heterogeneousnetworks which utilize similar timing mechanisms can be used. The aboveterm “audio signal” is intended to encompass signals communicatedbetween the various components in the network that contain audioinformation to reproduce the original audio signal sent from theoriginating End Device to the reproducing End Devices (e.g. compressedor encrypted signals that are based on, but are not exactly, theoriginal audio signal).

The techniques shown in FIGS. 1 and 2 coordinate audio or other mediareproduction across heterogeneous communication systems. Bysynchronizing the presentation time of audio to a group, collocated enddevices all present audio at roughly the same time, providing coherentreproduction of the original audio. Thus, multiple listeners hear thesame audio from multiple end devices simultaneously and fair access to agiven floor in half-duplex communication systems is provided as eachlistener is given the opportunity to attempt floor acquisition at aboutthe same instant in time. Either embodiment contains the ability todelay audio to each RAN of the heterogeneous system independently,thereby accommodating End Devices that have significantly longertransmission-to-reproduction delays.

It will be understood that the terms and expressions used herein havethe ordinary meaning as is accorded to such terms and expressions withrespect to their corresponding respective areas of inquiry and studyexcept where specific meanings have otherwise been set forth herein.Relational terms such as first and second and the like may be usedsolely to distinguish one entity or action from another withoutnecessarily requiring or implying any actual such relationship or orderbetween such entities or actions. The terms “comprises,” “comprising,”or any other variation thereof, are intended to cover a non-exclusiveinclusion, such that a process, method, article, or apparatus thatcomprises a list of elements does not include only those elements butmay include other elements not expressly listed or inherent to suchprocess, method, article, or apparatus. An element proceeded by “a” or“an” does not, without further constraints, preclude the existence ofadditional identical elements in the process, method, article, orapparatus that comprises the element.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the spirit andscope of the invention defined by the claims, and that suchmodifications, alterations, and combinations are to be viewed as beingwithin the scope of the inventive concept. Thus, the specification andfigures are to be regarded in an illustrative rather than a restrictivesense, and all such modifications are intended to be included within thescope of present invention. The benefits, advantages, solutions toproblems, and any element(s) that may cause any benefit, advantage, orsolution to occur or become more pronounced are not to be construed as acritical, required, or essential features or elements of any or all theclaims. The invention is defined solely by any claims issuing from thisapplication and all equivalents of those issued claims.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed embodiment. Thus the following claims arehereby incorporated into the Detailed Description, with each claimstanding on its own as a separately claimed subject matter.

1. A method of coordinating audio reproduction for heterogeneous enddevices in heterogeneous first and second networks, the methodcomprising: receiving an audio signal from one of the end devices;associating a first timestamp with a first audio stream and a secondtimestamp with a second audio stream, the first and second timestampsbeing different, each of the first and second audio streams containingaudio information of the received audio signal; and sending the firstaudio stream to a first end device in the first network and the secondaudio stream to a second end device in the second network, the first andsecond timestamps providing timing information such that the audiosignal is reproducible by the first and second end devices at asubstantially simultaneous time.
 2. The method of claim 1, wherein thefirst network is a narrowband simulcast network, the first end device isa narrowband end device, the first timestamp comprises a GPS-derivedsimulcast transmission timestamp, the second network is an IP-enabledbroadband network, the second end device is an IP-enabled broadband enddevice, and the second timestamp comprises a time-of-day reproductiontimestamp.
 3. The method of claim 1, further comprising delayingreproduction of the audio signal in the first network using the firsttimestamp and in the second network using the second timestamp, thereproduction being delayed to account for a longest delay in the firstand second networks, the longest delay being a longest time intervalbetween when the first and second audio streams are sent from acontroller to the heterogeneous end devices to when a last of theheterogeneous end devices reproduces the audio signal.
 4. The method ofclaim 3, wherein generation of the first and second timestamps occurwithin a single controller, further comprising the controllercalculating the first and second timestamps using an algorithm having: afirst quantity equaling a first delay between when the first audiostream is sent from a controller to when an intermediary responsible forforwarding the first audio stream to a last of first end devices in thefirst network receives the first audio stream plus a time to process thefirst audio signal in the last of the first end devices, a secondquantity equaling a second delay between when the second audio stream issent from a controller to when a last of second end devices in thesecond network receives the second audio stream plus a time to processthe second audio signal in the last of the second end devices, a thirdquantity equaling the product of N and the time represented by a singleframe of audio, where N is a monotonically increasing integer startingat 0 and incremented for each successive audio frame processed, and aninitial clock time initialized at the start of the first audio stream,in which in the algorithm: if the longest delay is a result of the delaybetween the controller and one of the end devices in the first network,then: the first timestamp is equal to the initial clock time plus thefirst quantity minus the time to process the first audio signal in thelast of the first end devices plus the third quantity, and the secondtimestamp is equal to the initial clock time plus the first quantityplus the third quantity; otherwise if the longest delay is a result ofthe delay between the controller and one of the second end devices,then: the first timestamp is equal to the initial clock time plus thesecond quantity minus the time to process the first audio signal in thelast of the first end devices plus the third quantity, and the secondtimestamp is equal to the initial clock time plus the second quantityplus the third quantity.
 5. The method of claim 3, wherein generation ofthe first and second timestamps occurs within first and secondcontrollers, respectively, the first and second controllers aredifferent, and the second controller adheres to the interface behavioursof a first base station in the first network.
 6. The method of claim 3,wherein generation of the first and second timestamps occurs withinfirst and second controllers, respectively, the first and secondcontrollers are different, the second controller provides a one-waydelay measurement to the first controller, and the one-way delaymeasurement is calculated using an algorithm having: a first quantityequaling a first delay between when the first audio stream is sent fromthe first controller to when the second controller receives the firstaudio stream, the second controller responsible for forwarding thesecond audio stream to a last of second end devices in the secondnetwork, and a second quantity equaling a second delay between when thesecond audio stream is sent from the second controller to when the lastof second end devices in the second network receives the second audiostream plus a time to process the second audio signal in the last of thesecond end devices, in which: the one-way delay measurement calculatedby the second controller and returned to the first controller is equalto the first quantity plus the second quantity minus a time to processthe first audio signal in the last of the first end devices.
 7. Themethod of claim 3 wherein generation of the first and second timestampsoccurs within first and second controllers, respectively, the first andsecond controllers are different, the method further comprisingcalculating the first and second timestamps using an algorithm having: afirst quantity equaling a first delay between when the first audiostream is sent from the first controller to when an intermediaryresponsible for forwarding the first audio stream to a last of first enddevices in the first network receives the first audio stream, a secondquantity equaling a second delay between when the first audio stream issent from the first controller to when the second controller receivesthe first audio stream, the second controller responsible for forwardingthe second audio stream to a last of second end devices in the secondnetwork, a third quantity equaling a third delay between when the secondaudio stream is sent from the second controller to when the last ofsecond end devices in the second network receives the second audiostream plus a time to process the second audio signal in the last of thesecond end devices, a fourth quantity equaling the second quantity plusthe third quantity minus a time to process the first audio signal in thelast of the first end devices, a fifth quantity equaling the larger offirst or fourth quantities, a sixth quantity equaling the product of Nand the time represented by a single frame of audio, where N is amonotonically increasing integer starting at 0 and incremented for eachsuccessive audio frame processed, and an initial clock time initializedat the start of the first audio stream, in which: the first timestamp isequal to the initial clock time plus the fifth quantity plus the sixthquantity, and the second timestamp is equal to the first timestamp plusthe time to process the first audio signal in the last of the first enddevices.
 8. The method of claim 3, further comprising adjusting thelongest delay to reproduce the audio signal whenever at least oneheterogeneous end device joins or leaves a group including the first andsecond end devices if the at least one heterogeneous end device affectsthe longest delay in a statistically significantly fashion.
 9. Themethod of claim 1, further comprising: calculating a delay from when aninitial signal is sent from a controller to a particular heterogeneousend device to when the particular heterogeneous end device reproducesthe initial signal for each of the heterogeneous end devices; discardingthe calculated delays that are in a statistically significant percentileof the delays calculated to establish a longest delay; and delaying thereproduction of the audio signal, using the first and second timestamps,to account for the longest delay.
 10. A network comprising: a firstnetwork having first base stations and first end devices; a secondnetwork having second base stations and second end devices, the firstand second end devices being heterogeneous; and a controller that:receives an audio signal from one of the first or second end devices,associates a first timestamp with a first audio stream and a secondtimestamp with a second audio stream, each of the first and second audiostreams containing audio information of the received audio signal, thefirst and second timestamps being different and providing timinginformation such that the audio signal is reproducible by the first andsecond end devices at a substantially simultaneous time, sends the firstaudio stream and the first timestamp to at least one of the first basestations, and sends the second audio stream and the second timestamp toat least one of the second base stations, the at least one of the firstbase stations transmitting the first audio stream to a first end deviceassociated with the at least one of the first base stations at a timeindicated by the first timestamp, the at least one of the second basestations transmitting the second audio stream to a second end deviceassociated with the at least one of the second base stations, and thesecond end device reproducing the audio signal contained in the secondaudio stream at a time indicated by the second timestamp.
 11. Thenetwork of claim 10, wherein the first network is a narrowband simulcastnetwork, the first base stations are narrowband simulcast base stations,the first end devices are narrowband end devices, the first timestamp isa GPS-derived simulcast transmission timestamp, the second network is anIP-enabled broadband network, the second base stations are IP-enabledbroadband base stations, the second end devices are IP-enabled broadbandend devices, and the second timestamp comprises a time-of-dayreproduction timestamp.
 12. The network of claim 10, wherein thecontroller further calculates and stores a longest delay forreproduction of the audio signal and accommodates the longest delayusing the first and second timestamps, the longest delay being a longesttime interval between when the first and second audio streams are sentfrom the controller to the first and second end devices to when a lastof the first and second end devices reproduces the audio signal.
 13. Thenetwork of claim 10, wherein the controller receives delay measurementsfrom the first base stations and second end devices and calculates thelongest delay using the delay measurements.
 14. The network of claim 10,wherein the controller further: calculates a delay from when an initialsignal is sent from the controller to a particular first or second enddevice to when the particular first or second end device reproduces theinitial signal for each of the first and second end devices; discardsthe calculated delays that are in a statistically significant percentileof longest delays calculated to establish a longest delay of thecalculated delays that have not been discarded; and stores the longestdelay for reproduction of the audio signal; and calculates the first andsecond timestamps using the longest delay.
 15. The network of claim 10,wherein generation of the first and second timestamps occur within asingle controller, the network further comprising a controllercalculating the first and second timestamps using an algorithm having: afirst quantity equaling a first delay between when the first audiostream is sent from the controller to when a first base stationforwarding the first audio stream to a last of the first end devicesreceives the first audio stream plus a time to process the first audiosignal in the last of the first end devices, a second quantity equalinga second delay between when the second audio stream is sent from thecontroller to when a last of the second end devices receives the secondaudio stream plus a time to process the second audio signal in the lastof the second end devices, a third quantity equaling the product of Nand the time represented by a single frame of audio, where N is amonotonically increasing integer starting at 0 and incremented for eachsuccessive audio frame processed, and an initial clock time initializedat the start of the first audio stream, in which in the algorithm: ifthe longest delay is a result of the delay between the controller andone of the first end devices, then: the first timestamp is equal to theinitial clock time plus the first quantity minus the time to process thefirst audio signal in the last of the first end devices plus the thirdquantity, and the second timestamp is equal to the initial clock timeplus the first quantity plus the third quantity; otherwise if thelongest delay is a result of the delay between the controller and one ofthe second end devices, then: the first timestamp is equal to theinitial clock time plus the second quantity minus the time to processthe first audio signal in the last of the first end devices plus thethird quantity, and the second timestamp is equal to the initial clocktime plus the second quantity plus the third quantity.
 16. The networkof claim 10, wherein the controller comprises a first controller and asecond controller, the first controller sending the first audio streamand the first timestamp to both the first base stations and to thesecond controller, the second controller generating the second timestampin response to receiving the first timestamp and sending the secondaudio stream and the second timestamp to the second end devices throughthe second base stations.
 17. The network configuration of claim 16,wherein the second controller adheres to the interface behaviours of afirst base station in the first network.
 18. The network configurationof claim 16, wherein the second controller provides a one-way delaymeasurement to the first controller, the one-way delay measurement iscalculated using an algorithm having: a first quantity equaling a firstdelay between when the first audio stream is sent from the firstcontroller to when the second controller receives the first audiostream, the second controller responsible for forwarding the secondaudio stream to a last of second end devices in the second network, anda second quantity equaling a second delay between when the second audiostream is sent from the second controller to when the last of second enddevices in the second network receives the second audio stream plus atime to process the second audio signal in the last of the second enddevices, in which: the one-way delay measurement calculated by thesecond controller and returned to the first controller is equal to thefirst quantity plus the second quantity minus a time to process thefirst audio signal in the last of the first end devices.
 19. The networkconfiguration of claim 16, wherein the first controller calculates thefirst and second timestamp delays using an algorithm having: a firstquantity equaling a first delay between when the first audio stream issent from the first controller to when a first base station responsiblefor forwarding the first audio stream to a last of first end devices inthe first network receives the first audio stream, a second quantityequaling a second delay between when the first audio stream is sent fromthe first controller to when the second controller receives the firstaudio stream, the second controller responsible for forwarding thesecond audio stream to a last of second end devices in the secondnetwork, a third quantity equaling a third delay between when the secondaudio stream is sent from the second controller to when the last ofsecond end devices in the second network receives the second audiostream plus a time to process the second audio signal in the last of thesecond end devices, a fourth quantity equaling the second quantity plusthe third quantity minus the time to process the first audio signal inthe last of the first end devices, a fifth quantity equaling the largerof first or fourth quantities, a sixth quantity equaling the product ofN and the time represented by a single frame of audio, where N is amonotonically increasing integer starting at 0 and incremented for eachsuccessive audio frame processed, and an initial clock time initializedat the start of the first audio stream, in which: the first timestamp isequal to the initial clock time plus the fifth quantity plus the sixthquantity, and the second timestamp is equal to the first timestamp plusthe time to process the first audio signal in the last of the first enddevices.